The present invention relates to a pull-back ram cylinder apparatus in combination with a vertical mold clamping machine to be incorporated in a vertical molding apparatus such as a vertical die casting machine.
In a mold clamping machine of a vertical die casting machine, a movable platen carrying a movable mold is brought down by a main ram, actuated by a clamping cylinder, to contact the movable mold with a stationary mold in such manner that the movable mold is registered with the stationary mold, and at the step of opening the molds, the movable mold is pulled up by utilizing a pull-back ram cylinder to effect the opening of the molds. In a vertical die casting machine having the above structure, it is indispensable for the movable platen to be moved down axially at a high speed just before the stationary mold comes into contact with the movable mold, and at the mold contacting step, the speed of the movable mold should be lowered. If such movement of the movable mold is not attained, the movable mold impinges violently against the stationary mold and both the molds and other members would be damaged. Various buffering means have been proposed for this purpose, and they are defective in that the structure is complicated and the speed of the downward movement of the movable mold cannot be sufficiently controlled. For example, in a conventional die casting machine, a limit switch is actuated to switch a valve, through which hydraulic pressure is applied, to another valve, for the same purpose, having a reduced degree of the opening for the pressure oil, so that the speed of the movable mold is reduced. Alternatively, a limit switch is actuated to reduce the degree of the opening of a single valve through which the hydraulic pressure is applied. Such a limit switch is complicated in that it is difficult to determine the timing of the switching. Further, in practice, troubles, in that the switching is not timed appropriately, sometimes occur. One such trouble is that the movable mold reaches the stationary mold at a high speed. This generates a strong force and damages the molds. Still further, even if the switching is well timed, a shock is produced every time the limit switch is actuated, since the speed of the movable mold is changed quickly, that is, is not changed gradually.
In order to attain the high speed mold clamping operation which is desired, there is provided, in the conventional machine, means for effecting a cushion between the main ram and the clamping cylinder when the movable mold approaches the stationary mold, so that the movable mold is buffered against the stationary mold. The cushion means consists of, in combination, a restricted lower end portion of the clamping cylinder and an enlarged and tapered upper end portion (head end portion) of the main ram, or a restricted lower end portion of the pull-back cylinder and a tapered head portion of the pull-back ram between the restricted lower and enlarged upper portions of the ram. With such cushion means, it is not permitted to design the tapered portion so as to have a long axial length, and, under the circumstances, it is difficult to determine an optimum angle of the tapered surface relative to the axis.